1. Field of the Invention
The present invention relates to a display apparatus, a device for driving the display apparatus, and a method of driving the display apparatus.
2. Description of the Related Art
Liquid crystal display (LCD) devices having small/medium display screen used for, for example, mobile phone uses charge pump because small power consumption and small size of driver integrated circuit (IC) are required.
FIG. 1A is a circuit diagram showing a general charge pump circuit, and FIGS. 1B and 1C are graphs showing on-off operations of the switches SWC1, SWC2, SWC3 and SWC4.
As shown in FIGS. 1A, 1B and 1C, when the first and second switches SWC1 and SWC2 for charging purpose are turned-on and the switches SWG1 and SWG2 for regulating purpose are turned-off, the first capacitor C1 is charged with the charges of the battery. Afterwards, when the first and second switches SWC1 and SWC2 are turned-off and the switches SWG1 and SWG2 are turned-on, the charges accumulated at the first capacitor C1 and the charges of the battery are charged at the second capacitor C2. The second capacitor C2 regulates (or smoothes) an output voltage signal VCP of the charge pump.
However, the output voltage signal VCP of the charge pump has ripples due to variation of an output load since the charge pump does not perform regulating operation according to the variation of the output load.
Liquid crystal display devices employ a capacitor and generate gray scale voltage signals (or data voltage signals), and the liquid crystal display devices employs a capacitor having a large capacitance so as to reduce the ripples. However, the capacitor having the large capacitance may induce increased size of the charge pump circuit of the liquid crystal display devices.
Further, even though the capacitor having the large capacitance is employed in the charge pump of the liquid crystal display device, the ripples may not completely be prevented. Thus, ripple (or water-fall) phenomenon is shown on the display screen of the liquid crystal display device. Especially, the ripple (or water-fall) phenomenon is serious in the liquid crystal display devices having small/medium display screen that is driven using line inversion method.
FIG. 2B is a graph showing a waveform of a voltage signal VCP output from a charge pump employed in a liquid crystal display device, and FIG. 2A is a graph showing a waveform of a voltage signal VD that is obtained after regulating the voltage signal VD of FIG. 2B.
The voltage signal VCP is a square wave signal generated from the charge pump. When a capacitor is coupled to the charge pump so as to regulate the square wave, a reference gray scale voltage signal VD of FIG. 2A is obtained. The waveform of the reference gray scale voltage signal VD of FIG. 2A has ripples. The capacitor coupled to the charge pump so as to regulate the square wave has a large capacitance.
The gray scale voltage signals (or data voltage signals) corresponding to ‘a’ point, ‘b’ point or ‘c’ point of the reference gray scale voltage signal VD may be applied to data lines of a liquid crystal display panel. Thus, it is not predictable which point (‘a’, ‘b’ or ‘c’, etc.) of the reference gray scale voltage signal VD will be shown as the gray scale voltage signals.
Especially, the ripple phenomenon is serious in middle gray scale levels in which the voltage deviation between gray scale levels is small. For example, when each of Red, Green and Blue data are expressed using 6 bits (total 18 bits for the RGB data), the gray scale levels of each of Red, Green and Blue data is 64 and the total number of color combination is 262,144 (26*26*26=262,144 colors, 26=64 gray scale levels).
In a middle gray scale level such as 32-gray scale level, when the gray scale voltage level corresponding to ‘a’ point of the reference gray scale voltage signal VD is difference from the gray scale voltage level corresponding to ‘b’ point of the reference gray scale voltage signal VD, the voltage deviation between gray scale levels is generated, and ripples are shown on the display screen.
Namely, even though the gray scale voltage signal has the same gray scale level, a gray scale voltage level corresponding to a point between ‘a’ and ‘b’ points of the reference gray scale voltage signal VD may be applied to a portion of the display screen and a gray scale voltage level corresponding to a point between ‘b’ and ‘c’ points of the reference gray scale voltage signal VD may be applied to another portion of the display screen, thus ripples are shown on the display screen.